Previous Gate Question Topic Wise Electrical Machines

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Previous gate question Topic Electrical Machines

1. The slip of an induction motor normally does not depend on

(A) rotor speed (B) synchronous speed

(C) shaft torque (D) core-loss component

Ans: Option (D) is correct. ns = synchronous speed n = rotor speed Thus, slip depend on synchronous speed and the rotor speed. Also, torque increases with increasing slip up to a maximum value and then decreases. Slip does not depend on core/loss component

2. A 4 point starter is used to start and control the speed of a

(A) dc shunt motor with armature resistance control

(B) dc shunt motor with field weakening control

(C) dc series motor

(D) dc compound motor

ans: Option (A) is correct.

3.A three phase, salient pole synchronous motor is connected to an infinite bus. It is operated at no load a normal excitation. The field excitation of the motor is first reduced to zero and then increased in reverse direction gradually. Then the armature current.

(A) Increases continuously

(B) First increases and then decreases steeply

(C) First decreases and then increases steeply

(D) Remains constant

Ans: Option (B) is correct. The Back emf will go to zero when field is reduced, so Current input will be increased. But when Field increases (though in reverse direction) the back emf will cause the current to reduce.

4. A field excitation of 20 A in a certain alternator results in an armature current of 400 A in short circuit and a terminal voltage of 2000 V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200 A is

(A) 1 V (B) 10 V

(C) 100 V (D) 1000 V

Ans Option (D) is correct. Given field excitation of = 20 A Armature current = 400 A Short circuit and terminal voltage = 200 V On open circuit, load current = 200 A So, Internal resistance = 2000/400 =5 Ω Internal vol. drop = 5x 200 = 1000 V.

5. Distributed winding and short chording employed in AC machines will result in

(A) increase in emf and reduction in harmonics

(B) reduction in emf and increase in harmonics

(C) increase in both emf and harmonics

(D) reduction in both emf and harmonics

Ans Option (D) is correct Distributed winding and short chording employed in AC machine will result in reduction of emf and harmonics.

6. In a stepper motor, the detent torque means

(A) minimum of the static torque with the phase winding excited

(B) maximum of the static torque with the phase winding excited

(C) minimum of the static torque with the phase winding unexcited

(D) maximum of the static torque with the phase winding unexcited

Ans Option (D) is correct. Detent torque/Restraining toque: The residual magnetism in the permanent magnetic material produced. The detent torque is defined as the maximum load torque that can be applied to the shaft of an unexcited motor without causing continuous rotation. In case the motor is unexcited.

7. The speed of rotation of stator magnetic field with respect to rotor structure will be

(A) 90 rpm in the direction of rotation

(B) 90 rpm in the opposite direction of rotation

(C) 1500 rpm in the direction of rotation

(D) 1500 rpm in the opposite direction of rotation

Ans Option (A) is correct. Given 3-φ, 440 V, 50 Hz, 4-Pole slip ring motor Motor is coupled to 220 V Ns =1500 rpm , Relative speed = 1500 − 1410 = 90 rpm in the direction of rotation.

8.In a transformer, zero voltage regulation at full load is

(A) not possible

(B) possible at unity power factor load

(C) possible at leading power factor load

(D) possible at lagging power factor load 

Ans : Option (C) is correct. In transformer zero voltage regulation at full load gives leading power factor.

9. The dc motor, which can provide zero speed regulation at full load without any controller is

(A) series 

(B) shunt

(C) cumulative compound 

(D) differential compound

Option (B) is correct.

10. A three-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half, with field current held constant, its new power factor will be

(A) unity

(B) leading

(C) lagging

(D) dependent on machine parameters

Ans : Option (B) is correct. When the 3-φ synchronous motor running at full load and unity power factor and shaft load is reduced half but field current is constant then it gives leading power factor

11. A three-phase, three-stack, variable reluctance step motor has 20 poles on each rotor and stator stack. The step angle of this step motor is

(A) 3c (B) 6c

(C) 9c (D) 18c 

Ans : Option (B) is correct. Given 3-φ, 3-stack Variable reluctance step motor has 20-poles Step angle = (360)/ (3x20)= 6

12. In transformers, which of the following statements is valid ?

(A) In an open circuit test, copper losses are obtained while in short circuit

test, core losses are obtained

(B) In an open circuit test, current is drawn at high power factor

(C) In a short circuit test, current is drawn at zero power factor

(D) In an open circuit test, current is drawn at low power factor

ans : Option (D) is correct. In transformer, in open circuit test, current is drawn at low power factor but in short circuit test current drawn at high power factor.